Biomimetic Total Syntheses of Cassiarins A and B

A rhodium-catalyzed C-H activation/cyclization approach toward the total syntheses of cassiarin C and 8-O-methylcassiarin A from a common intermediate

Three short and efficient total syntheses of cassiarin C are reported, from a chromanone common key intermediate. A C-H activation strategy, under rhodium catalysis on its pivaloyl oxime, enabled the installation of the pyridine ring. Dehydrogenation of 8-O-methylcassiarin C afforded 8-O-methylcassiarin A. A kinetic experiment and DFT calculations of the intermediates helped to gain insight into the unusual site- and stereo-specific H/D exchange of cassiarin C in CD3OD.

Quick Access to Protoberberine and Tetrahydroprotoberberine Derivatives/Analogues with One-Pot Sequential Isochromenylium-Mediated Annulation

A one-pot access to the protoberberine and tetrahydroprotoberberine scaffold has been developed from easily available primary amine-functionalized o-alkynylbenzaldehydes. The core skeleton of protoberberine was afforded via in situ generation of isochromenylium by Ag(I) catalysis, subsequent cyclization to isoquinolinium, and final hydride reduction by a continuous operation procedure. The newly developed step-economic protocol shows excellent regio- and stereoselectivity and is capable of achieving those previously unavailable derivatives/analogues with electron-deficient substituents, providing a robust tool for medicinal investigations of protoberberines and derivatives.

Alatains A and B, unique hetero-dimeric polyphenols from Cassia alata and their anti-tobacco mosaic virus activity

Two structurally unique polyphenols, alatains A (1) and B (2), were isolated from the bark of Cassia alata. Their structures were elucidated on the basis of spectroscopic analysis. Compounds 1 and 2 represent a new type of hetero-dimeric polyphenols with a C-14-C-5' linkage, biogenetically formed by an unusual intermolecular oxidative phenol-coupling reaction between a chromone unit and an isocoumarin moiety. Moreover, compounds 1 and 2 showed significant anti-tobacco mosaic virus (anti-TMV) inhibition IC50 values of 18.8 and 11.4 μM, respectively. Alatains A and B also exhibited promising protective effects on TMV infection of the host plants (Nicotiana tabacum) with the inhibition rates of 54.6% and 69.7% at the concentration of 20 μM, respectively. The results provided a new structural template for potential anti-TMV agent discovery.

First total synthesis of ampullosine, a unique isoquinoline alkaloid isolated from Sepedonium ampullosporum, and of the related permethylampullosine

A straightforward and convenient approach toward the first total synthesis of ampullosine, a structurally unique 3-methylisoquinoline alkaloid isolated from Sepedonium ampullosporum, is reported. Access to the related O-methyl ampullosine methyl ester from a common intermediate is also disclosed. The synthetic sequence toward the natural product comprised a Kolbe-type carboxylation of 3,5-dihydroxybenzoic acid and further esterification of the diacid, followed by masking of one of the phenols through selective ester reduction and subsequent acetonide formation. Installation of the three-carbon atom required for the 3-methylpyridine ring was performed by triflation of the remaining free phenol and a Pd-catalyzed Suzuki-Miyaura reaction with potassium E-propenyltrifluoroborate. Deprotection of the acetonide, followed by partial oxidation of the benzylic alcohol to the salicylaldehyde, O-methylation of the free phenol and hydrazonation of the resulting ortho-anisaldehyde derivative gave a hydrazone-based 1-azatriene. This was further subjected to 6π-azaelectrocyclization to afford permethylampullosine (11 steps, 14% overall yield), whereas exhaustive demethylation with AlI3 generated in situ gave ampullosine (12 steps, 3.2% global yield).

The 6π-azaelectrocyclization of azatrienes. Synthetic applications in natural products, bioactive heterocycles, and related fields

Covering: 2006 to 2018 The application of the 6π-azaelectrocyclization of azatrienes as a key strategy for the synthesis of natural products, their analogs and related bioactive or biomedically-relevant compounds (from 2006 to date) is comprehensively reviewed. Details about reaction optimization studies, relevant reaction mechanisms and conditions are also discussed.

Donor- and acceptor-enynals/enynones

Three kinds of transformations based on benzo-fused donor- and acceptor-enynals/enynones, including reactions with alkenes, alkynes, and H₂O, were discussed.

Synthesis of Natural Product-like Polyheterocycles via One-Pot Cascade Oximation, C-H Activation, and Alkyne Annulation

An efficient protocol for the direct transformation of chroman-4-ones to tricyclic fused pyridines with the skeleton of cassiarins, a family of alkaloids with antimalarial activity, was developed. Also, a general strategy for modular construction of polyheterocycles with diverse natural product-like skeletons was developed by using ketone-alkyne bifunctional substrates. These reactions involved a one-pot cascade oximation of ketones, rhodium-catalyzed C-H activation, and intermolecular/intramolecular alkyne annulations under mild conditions with high atom, step, and redox economy.

Convenient One-Step Synthesis of Benzo[c]phenanthridines by Three-Component Reactions of Isochromenylium Tetrafluoroborates and Stilbenes in Acetonitrile

A new type of three-component reaction of air-stable isochromenylium tetrafluoroborates with electron-rich stilbenes in acetonitrile has been developed under catalyst-free conditions in this work. This cascade multibond-formation reaction is initiated by an intermolecular oxa [4 + 2]-cycloaddition, relayed with a nucleophilic addition of acetonitrile, and terminated by an intramolecular Friedel-Crafts reaction, affording the corresponding benzo[c]phenanthridine analogues in one step.

Fistulains A and B, New Bischromones from the Bark of Cassia fistula, and Their Activities

Fistulains A and B (1 and 2), two novel bischromones with unique coupling patterns, alone with their biosynthetic related compound 3, were isolated from the bark of Cassia fistula. Fistulain A represents a new type of dimeric chromone alkaloid biogenetically derived from a chromone and a tricyclic alkaloid through an unusual C-14-N linkage. Fistulain B has a new carbon skeleton with a C-14-C-5' linkage formed between two different chromone units. Fistulain A displayed anti-TMV activity, and both 1 and 2 showed weak cytotoxicities.

Angular tricyclic benzofurans and related natural products of fungal origin. Isolation, biological activity and synthesis

Naturally-occurring angular tricyclic benzofuran/isobenzofuran derivatives of fungal origin and related compounds, in which two heterocyclic rings are fused to a central benzenoid moiety, are covered. Emphasis is placed on the structure of the compounds, together with their relevant biological activities, source microorganisms, country or region of origin and environmental conditions. In addition, proposed biosynthetic pathways, as well as the total syntheses of some of the compounds, including those that lead to structural revision or to correct stereochemical assignments, and related synthetic efforts, are discussed in detail.

Concise synthesis of 5-methoxy-6-hydroxy-2-methylchromone-7-O- and 5-hydroxy-2-methylchromone-7-O-rutinosides. Investigation of their cytotoxic activities against several human tumor cell lines

The synthesis of two novel 2-methylchromone-7-O-rutinosides is reported, and the in vitro biological activities of these compounds and their synthetic precursors have been investigated on the basis of their cytotoxicity against several human tumor cell lines. The synthesis features early stage assembly of the acidic labile glycosidic bond between sugar and 2-methylchromone aglycon under phase transfer catalyzed glycosidation conditions, whereas all the other standard glycosylation conditions specific to a wide array of rutinosyl donors bearing different anomeric leaving groups (e.g., SPh, OC(NH)CCl(3), Br, OH groups) failed to furnish any detectable products.